The present invention relates to alarm systems. More particularly, the present invention relates to alarm systems utilizing infrared sensors for sensing both motion and fire or smoke.
Infrared (IR) detectors are widely known in the arts of intrusion detection and in fire/smoke detection. These detectors have basically two forms: active and passive. Passive IR detectors generally employ a group of radiation sensors coupled through amplifiers to a logic circuit. The radiation sensors detect changes in ambient infrared radiation (temperature). Because the ambient is subject to changes such as might be caused, e.g., by a gust of wind, and which are not the result of an intruder, systems utilizing intrusion passive IR sensors must filter out these changes. In order to filter out background changes, pairs of phase opposed sensors are coupled in series (or parallel) so that an identical change in temperature sensed by both sensors simultaneously is cancelled through "common mode rejection". With this arrangement, an intruder moving across the field covered by the sensors produces different changes in temperature at each sensor, or the same change but at different times.
Passive IR sensors are also used as fire detectors. When used as fire detectors, the passive IR sensors are coupled to a circuit which monitors the rate of increase in ambient temperature. If the rate of increase in ambient temperature is above a certain preset value, a fire warning is activated. A similar arrangement can be used to detect freezing by measuring the rate of decrease in ambient temperature. A rather sophisticated group of passive IR sensors is disclosed in U.S. Pat. No. 4,052,716 to Mortensen for "Fire and Intruder Detection and Alarm Apparatus." Mortensen's device includes a plurality of passive IR sensors arranged in series and coupled to bridge circuits to mask irrelevant changes in ambient temperature and minimize false alarms when used in fire detection or intruder detection.
Active IR detectors include a radiation source and an IR sensor which is sensitive to interruptions in the radiation sensed from the source. These detectors are used as intrusion detectors by providing a "path" of radiation from the source to the sensor in a place where the path is likely to be crossed (interrupted) by an intruder. As a smoke detector, active IR sensors are placed where smoke is likely to interrupt the path of radiation between the source and the sensor.
Among the most used and most effective type of intrusion detection devices are the passive IR detectors which in recent years have replaced other types such as microwave or ultrasonic detectors. Detectors based on the teachings of U.S. Pat. No. 3,958,118 are ubiquitous in the alarm industry. These very sensitive IR detectors are used exclusively to detect motion of an intruder whose body heat is detected.
In recent years there has been a trend towards combining systems for detecting intrusion, smoke, fire, freezing, etc. Generally, combination systems are more economical than separate systems because labor costs are reduced when installing multiple systems at the same time. In addition, common components such as a common power source and/or common alarm can be used, whereby the separate detector systems are powered by the same power source and trigger the same alarm. An example of such a system is disclosed in U.S. Pat. No. 4,484,181 to Schwartz for a combination burglar and smoke alarm where a smoke detector is housed in a compact case which also includes a spring switch which closes upon the movement of a door to which the case is clamped. A single sounding device is activated by the smoke sensor or the burglar-detect switch. U.S. Pat. No. 4,321,591 to Vieweg, utilizes the same concept and provides an ultrasonic burglar alarm, a smoke/gas detector, and an alarm clock in a single housing with a common power supply.
While most of these combination systems are "portable", the combination system concept has been applied to fixed installations as well. U.S. Pat. No. 4,319,229 to Kirkor discloses separate sensors for heat, smoke and IR radiation mounted in a common housing and wired to a single common alarm or remote annunciator. A similar approach is taken in U.S. Pat. No. 4,446,454 to Pyle for a "Home Security System" which includes separate sensors for intrusion and fire/smoke which transmit signals to a single alarm system.
While each of the above-described systems has a well served purpose, the systems of the art have not been designed to optimally integrate the various components of the system. For example, even in the so-called combination systems, the disclosed arrangements require that either separate sources (in the case of active IR systems) or separate sensors (in the case of passive IR systems) be provided for each detection function such as intrusion, smoke/fire, etc. Of course, each source and sensor has its separate economic cost in terms of manufacture and installation and the additional support such as wire and power that each separate source or sensor requires.